Here is a list of other solutions that work with PayPal:

BillingOrchard
Online billing application designed with developers' needs in mind.

BillMyClients.com
BillMyClients.com makes it easy to create and send invoices and bills online or through the mail. Our self-service and custom solutions will streamline your invoicing and billing.

Innovate PayPal Payment Manager (IPPM)
Automate product delivery without IPN scripting by processing standard PayPal emails.

ModernBill
A web-based automated billing system for the web hosting industry.

MollyGuard
Payment solutions for events and email newsletters. Mollyguard Events allows event managers to easily collect registration fees from event attendees.

Payko
Online money orders with PayPal

PayPal Payment Link Creator - Re-Ware (for Mac)
Tool enables a PayPal Premier seller to send email invoices with full payment URLs automatically generated. For Mac users.

PerlBill
A client management solution for companies selling on the internet.

ProgrammingBids
RFQ Marketplace for programming projects.

Smarter Scripts Smart Search
Popular pay-per-click search engine supports PayPal.

WebProofs
Online order forms.

WishCast Wizard Wishlist
Wish list systems for e-commerce sites.